Spokesman Khaled Saleh, speaking to reporters in Istanbul on the second day of a coalition meeting there, said the opposition and the rebel Free Syrian Army (FSA) "are on the same side and we are fighting a common enemy".
"If we are going to be in Geneva, they are going to be part of that delegation. They have as much interest in a successful and free democratic Syria as we do."
The opposition coalition's gathering was meant to forge a common position on the Geneva talks, which world powers want held to find a negotiated solution to Syria's conflict.
Saleh said the Turkey-based coalition would send two delegations into Syria to discuss with FSA leaders and civilian groups the prospect of participating in the Geneva talks.
The coalition was also continuing to work on a statement setting out its formal position on the talks, he said.